Seismic waves carry energy away from an earthquake rupture. P and S waves travel through Earth’s interior and are called body waves. Love and Rayleigh waves travel mainly near the surface. Stations record the combined motion, which depends on the source, path through Earth and local ground beneath each instrument.
Quick answer
| Wave | Motion | Relative arrival | Where it travels |
|---|---|---|---|
| P | Compression parallel to travel | First | Solids and fluids |
| S | Shear perpendicular to travel | After P | Solids, not liquid outer core |
| Love | Horizontal side-to-side shear | Surface-wave train | Near the surface |
| Rayleigh | Rolling elliptical motion | Surface-wave train | Near the surface |
Arrival order is useful, but it does not turn one station into a complete earthquake solution.
Body and surface waves
P means primary because P waves normally arrive first. They alternately compress and dilate material along their direction of travel. S means secondary; S waves move material across the direction of propagation and travel more slowly.
When body waves interact with boundaries and the surface, they generate additional phases. Love waves produce horizontal shear. Rayleigh waves combine vertical and horizontal motion in a rolling pattern. Their amplitudes and periods vary with source depth, magnitude and Earth structure.
How stations record waves
A seismometer records motion versus time in one or more components. The output, a seismogram, may contain distinct arrivals as well as overlapping coda and noise. Processing measures phase time, amplitude, period and polarity.
No waveform represents the source alone. Rock layers refract and reflect waves; attenuation removes energy; basins can amplify selected frequencies. Instrument response must also be removed or accounted for before records from different sensors are compared.
How wave arrivals are used
The time between P and S arrivals generally increases with distance from the source. Several stations and a velocity model allow a locator to estimate origin time and hypocenter. Amplitudes, durations and broader waveforms feed different magnitude calculations.
Wave records also help identify depth phases, fault mechanism and rupture complexity. Products describing local intensity use ground-motion observations and models, not merely a wave-type label.
Limits and common mistakes
S waves are not universally “the damaging wave,” and P waves are not universally harmless. Actual motion combines frequencies, directions and phases, while building response varies. Surface waves can be prominent in shallow earthquakes but are not the only source of strong ground motion.
The P–S interval alone gives an approximate distance under a chosen model, not a unique latitude, longitude and depth. Station coverage and the velocity model remain essential.
Common questions
Why do P waves arrive first?
Their elastic propagation speed is higher than that of S waves in the same material.
Why do S waves not cross the liquid outer core?
Shear waves require material with shear rigidity; a liquid does not support that propagation in the same way.
Are tsunami waves seismic waves?
No. A tsunami is a series of water waves. An earthquake can generate both seismic waves in Earth and a tsunami in the ocean.